1. Field of the Invention
The present invention relates to a turning behavior control device of a vehicle.
2. Description of the Related Art
Conventionally, techniques to enhance the safety of a vehicle by stabilizing the vehicle, which is turning, have been developed. For example, Japanese Patent Publication No. 2007-131229A described later discloses a technique in which a difference of the drive force between the right and the left wheel is fed back according to a yaw rate of a vehicle and a braking force given to each wheel of the vehicle is also fed back. Further, electronic control LSD (Limited Slip Differential) of a center differential gear, in which a degree of the limitation of the differential between the front and the rear wheel is variable, is fed back according to the yaw rate of a vehicle.
However, in some cases, it is difficult to suppress a tendency of over-steering, which is caused in a turning vehicle, only by executing control so that a drive force of the inner turning wheel can be increased. That is, since a load given to the outer turning wheel is increased while the vehicle is turning and a load given to the inner turning wheel is relatively decreased, a gripping force of the inner turning wheel to grip a road surface, that is, a traction of the inner turning wheel is lowered. Therefore, even if a drive force given to the inner turning wheel is increased, the inner wheel slips and it is impossible to generate a sufficiently high moment for suppressing the tendency of over-steering in some cases. In this connection, the above phenomenon tends to occur in the case where the turning vehicle is being accelerated.
In the case where the turning vehicle is being decelerated, not only a traction of the inner turning wheel is lowered but also a load given to the front wheel is increased and a load given to the rear wheel is relatively decreased and a traction of the rear wheel is lowered. Accordingly, even when a drive force control is executed between the right and the left wheel on the rear side, it is impossible to generate a sufficiently high moment to suppress the generation of under-steering and over-steering in some cases.
In the case where under-steering is generated when a vehicle of 4-wheel drive is turning, it is possible to adopt a method in which the tendency of under-steering is suppressed by enhancing the turning property of a vehicle by weakening the limitation of the differential between the front and the rear wheel. However, according to this method, since a traction of the entire vehicle is decreased, the accelerating performance of the vehicle is lowered. That is, on the assumption that the rear wheel of the vehicle has slipped under the condition that a differential between the front and the rear wheel made by the center differential gear is not limited, the rear wheel is further rotated. Therefore, torque originally to be transmitted to the front wheel is transmitted to the rear wheel which is slipping. Accordingly, the vehicle is limited from being accelerated.
In order to solve the above problems, the present applicant proposed the following control technique which is described in Japanese Patent Publication No. 2007-131229A. By applying the yaw rate feedback control, a limitation of the differential between the front and the rear wheel is made by the front and rear differential limitation device and a drive force control between the right and the left wheel of the vehicle is made by the right and left wheel torque difference generating device and further the brake device is integrally controlled. Only when over-steering is suppressed, in parallel with the control of the drive force between the right and the left wheel of the vehicle, controlling is executed so that a restricting force generated by the differential control between the front and the rear wheel made by the central differential gear is strengthened.
However, in the case where the yawing control is made by a braking force generated by the braking device (four wheel independence braking device) in the manner executed by the conventional constitution, a disadvantage of the feeling of speed reduction is caused. Especially when a braking force is given at the time of acceleration, control is made in the opposite direction to that of the driver's will in which the driver wants to accelerate. Therefore, the feeling of speed reduction becomes remarkable. In the case where over-steering is suppressed at the time of sudden acceleration, since the front wheel, the lateral force of which is reduced by a drive force, is braked and the lateral force is restored, over-steering is facilitated on the contrary. In the case where the vehicle is running at a low speed, deviation of the yaw rate is increased. Therefore, in the case of carrying out the suppression of under-steering and over-steering, the suppression is excessively made.
In the case where the yawing control is made by a right and left wheel torque difference generating device like the conventional constitution, since a load given to the rear wheel is decreased at the time of deceleration, a controlling capacity is lowered. Further, the suppression of over-steering made by a torque difference between the right and the left wheel on the rear side facilitates over-steering by a reduction of the lateral force of the rear wheel on the contrary.
In the case where the yawing control is executed by the front and rear differential limiting device, a vehicle, the behavior of which shows a tendency of under-steering, is put into a state of head-in/head-out depending upon the circumstances. Therefore, it is difficult to grasp a torque moving direction. Accordingly, it is difficult to suppress under-steering by the differential limitation made by the front and rear differential limiting device. Since the vehicle, the behavior of which shows a tendency of over-steering, is always put into a state of head-out, by the differential limitation made by the front and rear differential limiting device, the front wheel is given a drive force and the rear wheel is given a braking force. Therefore, when the differential limitation for suppressing over-steering is executed at the time of deceleration, the rear wheel, which is being braked, is further braked, which is a factor of facilitating over-steering.